Method and apparatus for color electrophotography

Abstract

A charging process, an exposure process, and a development process are performed to form a toner image on a photosensitive member for each of a plurality of different colors. The toner images for the respective colors form a composite color image. Second color toner image forming processes are effected subsequently to first color toner image forming processes. The first color charging process charges the photosensitive member to a first potential. The second color charging process charges the photosensitive member to a second potential. The second potential is set equal to or lower than the first potential. The composite color image is transferred to a sheet.

What is claimed is: 
     
       1. A method of color electrophotography comprising the steps of: (a) performing a charging process, an exposure process, and a development process to form a toner image on a photosensitive member for each of a plurality of different colors, with the resulting toner images for the respective colors forming a composite color image, wherein yellow toner image forming processes are effected subsequent to black toner image forming processes, wherein the black charging process charges the photosensitive member to a first potential, the yellow charging process charges the photosensitive member to a second potential and the second potential is equal to or lower than the first potential, and wherein the charge on the photosensitive member resulting from the yellow toner image forming process is not removed by light exposure after the yellow toner image is formed; and   (b) transferring the composite color image to a sheet.   
     
     
       2. The method of claim 1 wherein black, yellow, magenta, and cyan toner images are successively formed. 
     
     
       3. The method of claim 1 wherein the yellow development process comprises forcing toner to fly with a dc electric field. 
     
     
       4. The method of claim 1 wherein the second potential is in the range of 300 V to 700 V. 
     
     
       5. The method of claim 4 wherein the first potential is higher than the second potential. 
     
     
       6. The method of claim 1 wherein a charged potential of the photosensitive member is equal to or higher than 700 V during third color toner image forming processes which follow the yellow toner image forming processes. 
     
     
       7. The method of claim 1 further comprising the step of removing charges from the photosensitive member after the black toner image forming processes but before the yellow charging process. 
     
     
       8. The method of claim 7 wherein the charge removing step comprises using light to remove the charges from the photosensitive member. 
     
     
       9. The method of claim 7 wherein the charge removing step comprises using ac corona to remove the charges from the photosensitive member. 
     
     
       10. A method of color electrophotography comprising the steps of: (a) performing a charging process, an exposure process, and a development process to from a toner image on a photosensitive member for each of black, yellow, magenta, and cyan, with the resulting toner images for the respective colors forming a composite color image, wherein the black development process is of a noncontact type and each of the yellow development process, the magenta development process, and the cyan development process forces toner to fly with a dc electric field, wherein the black toner image forming processes are effected first and the yellow toner image forming process are effected second, wherein a first charged potential of the photosensitive member in the black image forming processes is equal to or higher than 700 and charges are removed from the photosensitive member after the black toner image forming processes, and wherein after the charge removing step, a second charged potential of the photosensitive member in the yellow image forming processes is in the range of 300 V to 700 V, and wherein the charge on the photosensitive member resulting from the yellow toner image forming processes is not removed by light exposure the yellow toner image is formed; and   (b) transferring the composite color image to a sheet.   
     
     
       11. An apparatus for color electrophotography comprising: (a) means for performing a charging process, an exposure process, and a development process to from a toner image on a photosensitive member for each of black, yellow, magenta, and cyan, with the toner images for the respective colors forming a composite color image, wherein the means for performing the black development process is of a noncontact type, and wherein each of the means for performing the yellow development process, the magenta development process, and the cyan development process forces toner to fly with a dc electric field;   (b) means for transferring the composite color image to a sheet;   (c) means for effecting the black toner image forming processes first and the yellow toner image forming processes second;   (d) means for setting a first charged potential of the photosensitive member in the black toner image forming processes equal to or higher than 700 V;   (e) means for removing charges from the photosensitive member after the black toner image forming processes; and   (f) means for, after the charge removal, setting a second charged potential of the photosensitive member in the yellow image forming processes in the range of 300 V to 700 V.   
     
     
       12. An apparatus for color electrophotography comprising: (a) a photosensitive member;   (b) means for charging the photosensitive member to a first potential in a black charging process;   (c) means for, after the first charging process, exposing the photosensitive member to light representative of black information in a first exposure process, thereby forming a first electrostatic latent image corresponding to the black information on the photosensitive member;   (d) black toner;   (e) means for developing the first electrostatic latent image into a black toner image by use of the black toner in a first development process;   (f) means for charging the photosensitive member to a second potential in a yellow charging process after the first development process, the second potential being equal to or lower than the first potential;   (g) means for exposing the photosensitive member to light representative of yellow information in a second exposure process after the second charging process, thereby forming a second electrostatic latent image corresponding to the yellow information on the photosensitive member;   (h) yellow toner; and   (i) means for developing the second electrostatic latent image into a corresponding yellow image by use of the yellow toner in a second development process, wherein the black and yellow toner images form a composite color toner image.   
     
     
       13. The apparatus of claim 12 wherein the first potential is equal to or higher than 700 V and the second potential is in the range of 300 V to 700 V. 
     
     
       14. The apparatus of claim 12 further comprising means for removing charges from the photosensitive member after the first development process but before the yellow charging process.